The keypad in cellular phones or PCS terminals is typically illuminated by light emitting diodes "LEDs," which emanate light in response to a current flow. Many other electronic devices also employ such LEDs for illumination purposes. In many instances these LEDs are employed in a battery-operated device. For example, cellular phones or PCS terminals are typically driven by a battery pack that provides a voltage signal level, V.sub.BAT, ranging around 4-6 volts, depending on the charge state of the battery pack.
A light emitting diode typically maintains a potential drop of about 2 volts across its terminal. The brightness of the LED depends on the current flow through it. Typically, a 10 mA current flow allows for an acceptable brightness level. Thus, many prior art systems employ few LEDs connected in parallel and powered by a battery pack voltage level, such as 4-6 volts. A typical PCS terminal or a cellular phone such as 30, illustrated in FIG. 2, requires up to 8 LEDs for proper brightness across all keys 36 and display screen 32. As a result, these prior art systems employ 8 branches of LEDs each drawing approximately 10 mA of current, leading to a total current discharge of about 80 mA. For a battery driven system, such a level of current discharge may lead to a shorter usage time between periodic charging of the battery.
In order to conserve battery power, other prior art systems, such as 10, employ two LEDs connected in series in each branch, as illustrated in FIG. 1. For example, as illustrated, eight LEDs are employed in four branches. The total current drain for the system illustrated in FIG. 1 is about 40 mA, which is half the amount of current drain of single-LED-per-branch systems. A battery pack 12 is coupled to four branches of LEDs such as 18 and 20, through a control resistor 16. A switch 22 is employed to activate the LEDs in each branch. The amount of current drawn in each branch depends on the value of resistor 16 and voltage fluctuations across battery pack 12. However, since there are now two LEDs coupled in series in each branch, the voltage drop across each branch is about 4 volts in addition to the voltage drop across switch 22, which is about 0.2 volts. The voltage drop across resistor 16 is the difference between the voltage level at the battery pack and the voltage level at each branch.
A shortcoming with this arrangement is that small voltage fluctuations at the battery pack may lead to relatively large current fluctuations across each branch. For example, as soon as the voltage level at the battery pack begins to decrease, the change in the LEDs' brightness may be noticed. Another disadvantage with the prior art LED drivers is that in cellular phones or PCS terminals, during a standby mode the battery voltage level is higher than during the talk mode. Thus, as soon as a call is received, the battery voltage level drops leading to a significant drop in the LED currents, resulting in a noticeable change in brightness. Another disadvantage is variations in the parameters of the switch, which usually comprises a switching transistor. Variations in temperature and transistor characteristics may lead to variations in voltage drop across the transistor, leading in further current fluctuations.
A further disadvantage is particularly prevalent in time division multiple access "TDMA" systems, where the cellular phone transmits signals at appropriate time slots. During such signal transmission, the battery voltage level drops, leading to significant current fluctuations across the LEDs, resulting in LED flickering.
Thus, there is a need for an LED drive circuit in a battery operated system, that is capable of maintaining a constant current flow through the LEDs for a wide range of power supply voltage signal levels.